Mast cells are primary effector cells of immediate hypersensitivity, and may be involved in host defense, tissue homeostasis and other forms of hypersensitivity. The current proposal extends work that originated with the identification of two types of human mast cells based on their protease phenotypes and the finding that stem cell factor enables human mast cells to develop in vitro from hematopoietic progenitors. Aim 1 examines new functional and phenotypic characteristics that distinguish MCT (tryptase+/chymase-) from MCTC (tryptase+/chymase-)types of human mast cells. These include: for MCTC cells- functional expression of cell-surface CD88, IL-6-mediated upregulation of chymase protein/mRNA in only MCTC cells, and proliferation of only MCTC cells in serum-free media; for MCT cells- mechanism by which IL-6makes MCT cells resist apoptosis to IL-4, failure of lung MCT cells to convert to MCTC cells, and evaluation of new genes expressed in only MCT cells (including their expression in nasal and bronchial mucosa of atopic rhinitis and asthma subjects). Aim 2 examines the activation of skin-derived MCTC cells through FcyRIIA as well as FceRI. Degradation of cytokines by proteases released from the same MCTC cells will be studied in vitro, and the in vivo consequences considered. Whether IgG immune-complexes activate human MCTC cells will be determined in vitro. Finally, the functional impact of increased FceRI levels on mature MCTC cells caused by IgE will be assessed in vitro as well as in vivo. Aim 3 will test whether Th1 and Th2 type cytokines modulate mediator production by human mast cells. The mechanisms by which Th1 and/or Th2 cytokines induce LTC4 and enhance PGD2 production by activated skin MCTC cells will be examined. Whether FcyRI expression by IFN-y-primed MCTC cells enhances the response to IgG immune complexes or to CRP will be determined. Finally, the functional impact of upregulating cytokine receptors such as IL-4R on skin MCTC cells will be assessed. These studies will extend known phenotypic differences between different types of human mast cells to important functional features, reveal new paradigms for mast cell activation, and lead to a better understanding of their role(s) in human disease.